LoRa Martin Heusse LIG / Drakkar
Lora in the ISM bands channel spacing : 200kHz • 433MHz Band ✓ Max Tx power 10dBm • EU 863-870MHz Band ✓ Max Tx power : 20dBm, by default 14dBm ✓ Rx channels for the gateways 12 Modulation Bandwidth [kHz] Channel FSK Nb Duty Frequency Bitrate or Channels cycle [MHz] LoRa DR / Bitrate LoRa 125 868.10 DR0 to 3 <1% 868.30 DR5 868.50 / 0.3-5 kbps 13 Table 12: EU863-870 default channels ✓ Duty cycle is computed per sub band ✓ Each gateway may listen to 16 canaux in parallel. Specified to the devices when they associate LoRa — page 2 — transp. 2 TimeOnAir Toff ������� = − TimeOnAir DutyCycle ������� – LoRa™ Alliance ht to c
� – Lora in the ISM bands channel spacing : 200kHz (cont.) 18 in the EU863-870 band: DataRate Configuration Indicative physical TXPower Configuration bit rate [bit/s] 0 LoRa: SF12 / 125 kHz 250 0 20 dBm (if supported) 1 LoRa: SF11 / 125 kHz 440 1 14 dBm 2 LoRa: SF10 / 125 kHz 980 2 11 dBm 3 LoRa: SF9 / 125 kHz 1760 3 8 dBm 4 LoRa: SF8 / 125 kHz 3125 4 5 dBm 5 LoRa: SF7 / 125 kHz 5470 5 2 dBm 6 LoRa: SF7 / 250 kHz 11000 6..15 RFU 7 FSK: 50 kbps 50000 8..15 RFU 19 Table 14: Data rate and TX power table • Real world range : a few km NLOS, ≈ 20 km with LOS • Payload max size : from 59 to 230 B ( for datarate 4 and higher) LoRa™ Alliance LoRa — page 3 — transp. 3
ISM 868MHz band http://www.arcep.fr/uploads/tx_gsavis/14-1263.pdf http://www.anfr.fr/fileadmin/mediatheque/documents/tnrbf/TNRBF_ Ed2013_Mod8_-_Version_du_19_février_2016.pdf EIRP: 14dBm Freq. Duty cycle other uses 863-865 MHz 0,1 % Cordless microphones 865-868 MHz 1% RFID – ?? 868-868,6 MHz 1% (802.15.4 Sub-GHz) 868.6-868,7 MHz — Alarms 868,7-869,2 MHz 0,1% 869,2-869,7 MHz — Alarms 869,7-870 MHz 1% air force LoRa — page 4 — transp. 4
ERC Recommendation 70-03 http://www.erodocdb.dk/docs/doc98/official/pdf/rec7003e.pdf Sub band Freq. (MHz) Power Duty cycle BW (MHz) h1.3 863-870 14 dBm 0.1% 7 h1.4 868-868.6 14 dBm 1% 0.6 h1.5 868.7-869.2 14 dBm 0.1% 0.5 h1.6 869.4-869.65 27 dBm 10% 0.25 h1.7 869.7-870 7 dBm 100% 0.3 h1.7 869.7-870 14 dBm 1 % 0.3 Duty cycles are computed per sub-band : a device may consume 1% in h1.4, 10% in h1.6, 1% in h1.7, during the same hour for instance h1.4 encompasses the 3 defaults LoRa channels , h1.6 is used by the GW to respond to the devices ( cf. RX2) LoRa — page 5 — transp. 5
Transmissions • Classe A ( A ll devices) ✓ Exchange always initiated by the device Aloha access ✓ 2 rx windows follow the transmission at +1 s (same channel as TX) and +2 s (channel and SF fixed in advance) By default : RX2 at 869.525 MHz (center of h1.6), DR0 (SF12, 125 kHz) ✓ Each frame carries the Confirmed bit: (expecting and ACK) or unconfirmed • Classe B : B eacons Device listen periodically to beacons. Regular downlink slots are defined relative to the beacon • classe C : C ontinuous reception LoRa — page 6 — transp. 6 ‘s LoRa™ Alliance the rig
LoRaWAN Network Server App Server Concentrator GW LoRa / JSON / UDP SSL/TCP/IP App Server Concentrator GW App Server LoRa — page 7 — transp. 7
LoRaWAN (cont.) 6 Radio PHY layer: Preamble PHDR PHDR_CRC PHYPayload CRC* 7 Figure 5: Radio PHY structure (CRC* is only available on uplink messages) 1B … 4B 8 PHYPayload: MHDR MACPayload MIC 9 Figure 6: PHY payload structure 0 or 1B … 10 MACPayload: FHDR FPort FRMPayload 11 Figure 7: MAC payload structure 4B 1B 2B 0…15B 12 FHDR: DevAddr FCtrl FCnt FOpts 13 Figure 8: Frame header structure LoRa — page 8 — transp. 8 LoRa™ Alliance
LoRaWAN (cont.) • The frames only carry a single address, the device source/destination • Application demultiplexing : “FPort” (0: pure MAC command) • Piggybacking of MAC commands (power, data rate, channels, device state, rx delay 1 … ) in the will typically get several copies of the same frame (they have a seq. number) The net. server selects the best GW for a reply (if applicable) • In the core networks, the frames are forwarded with quite a bit of ancillary data (power, timestamp…) LoRa — page 9 — transp. 9 1 RX2 is always 1 s behind RX1
Activation • ABP — Activation By Personalization • OTAA — Over-The-Air Activation ✓ DevAddr allocation: the DevAddr is composed of 7 bits of Network ID and then a device-specific addr. (The DevAddr is assigned by the guest network. The real / immutable device identifiers are its NetEUI and AppEUI, which are stored in the device) ✓ Computation of the session keys: AppSKey, NetSKey, from the AppKey (128 bits) stored in the device LoRa — page 10 — transp. 10
What is a chirp ? CSS : Chirp Spread Spectrum • A linear frequency sweep/ramp − BW 2 < f < BW 2 1.0 0.5 tx(x) 0.0 −0.5 −1.0 −4 −2 0 2 4 t • Used by radars, bats, dolphins… LoRa — page 11 — transp. 11
Coding information on a chirp • It is the start freq. offset that codes the information (line 514) LoRa — page 12 — transp. 12
Reception • Multiplication of rx signal with a complex conjugate chirp (down chirp) e 2 π jt [ f 0 +( at + b ) mod BW ] × e − 2 π jt [ f 0 +( at ) mod BW ] = e 2 π jt [ b mod BW ] 2 SF → sweep BW in time 2 SF N.B. : a = BW 2 BW LoRa — page 13 — transp. 13
Reception (cont.) • if both chirps are in sync, we get a constant, otherwise LoRa — page 14 — transp. 14
FFT-based reception • FFT after sampling at rate BW • The symbol duration is N/BW → N samples • By frequency aliasing, a single frequency appears in the FFT ! LoRa — page 15 — transp. 15
Spread spectrum • Spreading factors from 7 to 12 ⇔ N goes from 2 7 to 2 12 , 7 to 12 bits per symbol • The bigger the SF the longer the chirp — 33 ms @ SF12. For LoRa, the preamble is also proportional to the SF ✞ ☎ R b = SF × BW The actual SF dynamics are ≈ 20 2 SF ✝ ✆ • Error correcting codes R = 4 5 • The actual max. SF is ≈ 340 ( 2 12 /12 ), so a transmission may √ survive a collision with a node closer by a ratio of ≈ 340 LoRa — page 16 — transp. 16
Frame sizes • Depends on SF : 51B payload at SF12, 242 at SF8 and SF7… • 51B @ SF12 → 1,3 s of continuous transmission! LoRa — page 17 — transp. 17
Initial Synchronisation ∂ f, ∂ t Receiver : f Preamble Data f+BW/2 f-BW/2 2 inverted chirps t N/BW • The device may quickly assess if there is a transmission → short rx1 and rx2 windows • The inverted chips in the preamble allow to find the two unknown variable the transmitter frequency and the relative time reference LoRa — page 18 — transp. 18
Localization • The observed δ t at several GW allow to compute relative time of arrival • Trilateration • Time ref. from GPS at the GWs LoRa — page 19 — transp. 19
A few remarks • A receiver can receive at several SF simultaneously ( ≈ 30 dB ) 2 . • It needs as many reception circuits as there are SFs 7 channels (6 CSS + 1 FSK) on all LoRa GWs • Localization is a by product of PHY initial sync. • Cell breathing ✓ Having more GWs allows : ▶ Lower the SF for closer devices ▶ lower the power 2 C. Goursaud & J.M. Gorce : “Dedicated networks for IoT : PHY / MAC state of LoRa — page 20 — transp. 20 the art and challenges”
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